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274

Templates and Jigs.



113

measure length for eccentric rod between pin of radius link and
edge of sheave. Reversing the crank, as at x, the valve is moved
to the front by the same amount, z, the length again obtained,
and the two averaged. In our description of the machining of
these rods we supposed the length already given; but it is always
found for the smith in this way, though often the rods are finished
in two pieces, and afterwards welded to correct length.

Put the valve rods in place, also the crosshead, connecting
rod, gudgeon, and slide blocks \ connect up to crank pin, having
previously fitted the brasses to the pin by scraping, and bolt down
tfre top slide bar with distance pieces between. Fix the regulator
valve box (previously put together), the cylinder cocks and lubri-
cators, the steam chest cover, and the back cylinder cover, making
all joints with red-lead ' putty' between. The putty is a mixture
of red and white lead, softened with boiled linseed oil. After
covering the joint surface, a piece of soft hemp line is laid once
or twice round, and the cover then put on. Portland cement or
asbestos discs are also used.

The last stage of all is to carry away the parts to their per-
manent position, and bolt down the-whole to its stone bed;
connect up the steam and exhaust pipes, and get up steam.

We shall now conclude with one or two general points.
^ Templates and Jigs.—The former have been sufficiently
explained in Figs. 253, 264, and 266. They are used very
extensively in much repeated work, thus saving a great deal
of time humarking off, and they take a variety of shapes. Jigs
are an extension of the template principle. Instead of thin
plates, castings of an inch or so in thickness are used, supplied
with holes where needed, the object being to guide the ilrill to its
proper place on the work without the necessity of lining-out at
all An example of the application of this principle to a cylinder
cover is shewn at Y, Fig. 273. (Set Appendix //.,/. 820.)

Hobbing a Worm Wheel.—A cutter for forming spur*
wheel teeth was given at Fig. 186, and a method of cutting bwel
teeth at Fig, 262, Worm-wheel teeth can also.be cut by first
turning in the lathe a worm of the correct shape, and of good
steel. This is then fluted to form a milling cutter, and is termed
a fob in the workshop. The operation is then much the same as

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